Positron emission tomography, PET for short, is an imaging method for showing the spatial distribution of a radioactive substance in an examination object. The radioactive substance used is a positron-emitting radionuclide. On collision with an electron the emitted positrons are converted to two photons moving away from one another in opposing directions. These are detected using a detector ring disposed around the examination object. If detection takes place within a predetermined time segment, this is judged to be a coincidence and therefore an annihilation event. The line connecting the detecting segments of the detector ring is referred to as the line of response or LOR for short. As the distance between positron emission and collision is short, it is determined that the emission source, in other words a point where parts of the radionuclide are located, is on or close to a LOR.
An individual annihilation event or a single LOR does not permit conclusions about a spatial distribution. It is only possible to calculate a positron emission tomography image dataset from the individual LORs by recording a number of annihilation events. The LORs can also be shown graphically in the time sequence in which they occurred in the form of a so-called sinogram. The precise calculation of a sinogram and the determination of a positron emission tomography image dataset therefrom is described for example in Fahey F. H., Data Acquisition in PET Imaging, J Nucl Med Technol 2002; 30:39-49.
In the following, the acquisition of a positron emission tomography image dataset refers to the spatially resolved recording of annihilation events with subsequent calculation of the positron emission tomography image dataset.
The acquisition time varies as a function of the radioactivity of the radionuclide and the desired signal intensity but it is approximately at least one minute.
Such acquisition times give rise to the problem of the examination region or examined patient moving. The emission source in the form of the radionuclide, which is generally packaged as a radiopharmaceutical and has been metabolized, is then of course also displaced so the determined positron emission tomography image dataset is blurred. Such blurring is therefore a motion artifact.
To avoid blurring it is known to acquire anatomy image datasets using a magnetic resonance device or computed tomography device parallel to the PET measurement. The image datasets are then used to calculate displacement vectors, which are transferred to the LORs in order to minimize the motion artifacts.
However this procedure is relatively time-consuming and computation-intensive, which is why a “real time” reconstruction of a positron emission tomography image dataset is not performed in this manner. It is however desirable for example when performing interventions.
The designs and embodiments also always apply similarly to single photon emission computed tomography (SPECT) acquisitions and image datasets. In contrast to PET, gamma emitters are used, which are detected with collimators. There are further differences, which are however generally not of relevance in respect of the inventive method. Reference is therefore made primarily to PET in the following for the sake of simplicity.